In view of environmental problems such as global warming and air pollution, reductions in VOC are now being carried on in various fields such as paints, inks and construction materials. Taking the field of paints, the development of paints indiverse forms, for example, powered paints, high-solids paints and water-based paints as environment-responsive paints is being carried on. In particular, water-based paints are under active development, resulting in the development of low-VOC paints and zero-VOC paints in attempts to reduce VOC. The term “low-VOC paint” means a water-based paint the VOC content of which is 0.1 wt % or lower, while the term “zero-VOC paint” means a water-based paint the VOC content of which is lower than 0.01 wt %, in other words, 100 ppm. A great deal of work is, therefore, under way to reduce the contents of VOC in resin dispersions to be used in paints, such as binder solutions and emulsions, and also to reduce the contents of VOC in film-forming aids and the like which may become VOC. It is also considered necessary to reduce the contents of VOC in dispersants to be used in such paints.
In a high-concentration dispersion of a pigment, a dispersant is used to keep the pigment dispersed. Due to VOC contained in an aqueous solution of the dispersant, a great deal of VOC is contained in the pigment dispersion. It has, therefore, been unable to use conventional pigment dispersions in low-VOC paints or zero-VOC paints from environmental consideration. There is, hence, an outstanding demand for a reduction in VOC, particularly a pigment dispersion usable in low-VOC paints or zero-VOC paints.
Surfactants are employed as a sort of dispersants having common utility. Using these surfactants, pigment dispersions substantially free of VOC have also been developed. Surfactants are, however, low in molecular weight and high in hydrophilicity, so that coatings formed from paints, which contain pigment dispersions making use of these surfactants, are poor in waterproofness. Accordingly, pigment dispersants of high molecular weight type have been developed. By providing a dispersant with a high molecular weight, a coating with the dispersant contained therein can show good waterproofness. Further, a binder and a dispersant to be used in a paint are required to have good compatibility with each other. A dispersant equipped with these properties in combination is thus needed.
As a result of an initial investigation, it was found that, although acrylic-resin-based dispersants are suited as dispersants for pigments, the dispersants can be hardly reduced in VOC. As a reason for this, an acrylic dispersant is obtained by solution polymerization. Even by a method that includes forming the dispersant into an aqueous solution and then heating the aqueous solution to distill off the solvent and residual monomers as VOC from the solution, it is difficult to completely eliminate VOC from the aqueous solution of the dispersant because of effects of the boiling point of water and that of VOC and their azeotropic relation. As a consequence, a great majority of VOC still remains in the dispersant.
There is another method that pours the above-described solution of the dispersant into a poor solvent for the dispersant, such as water, and has the dispersant precipitated in the solvent to remove VOC components from the dispersant. In this case, the dispersant is hydrophobic so that particles of the dispersant aggregate into a solid, bulky or powder form in the poor solvent, and therefore, VOC is confined to the particles of the dispersant. VOC cannot be eliminated even when the dispersant is washed. Even when the dispersant is dried, it is still difficult to completely eliminate VOC from the dispersant.